Soft-switching power converter

ABSTRACT

A pulse width modulated soft-switching power converter, having a transformer with a primary winding and a secondary winding, a secondary circuit coupled to the secondary winding, and a pair of main switches and a pair of auxiliary switches coupled to the primary winding. The main switches and auxiliary switches intermittently conduct an input voltage source to the primary winding of the transformer to operate the soft-switching power converter in four operation stages in each switching cycle. The main switches conduct the input voltage source to the transformer in a first operation stage. In a second operation stage, the conduction is cut off. The transformer operates as an inductor with the auxiliary switches switched on under zero-voltage or zero-current switching mode in a third operation stage. In the fourth operation stage, the auxiliary switches are switched off to achieve zero-voltage transition.

BACKGROUND OF INVENTION

[0001] Field of Invention

[0002] The present invention relates in general to a pulse width modulated power converter, and more particularly, to an improved pulse width modulated power converter using zero-voltage switching technique.

[0003] Power converters have been used to convert an unregulated power source to a regulated voltage or current source. The transformer that comprises a primary winding and a second winding is at the heart of most power converters. Typically, a switching device is connected to the primary winding to control energy transferred from the primary winding to the second winding and output therefrom. Currently, under the control of the switching device, the pulse width modulated power converter can be operated to at a high frequency with reduced size and weight. However, such a power converter suffers from the issues of switching loss, component stress and noise, and electromagnetic interference (EMI).

[0004] To resolve the switching loss problem of the pulse width modulation power converter, a phase-shift scheme for soft switching has been proposed, particularly for the high-frequency power conversion. For example, the full-bridge (FB) quasi-resonant zero-voltage switching (ZVS) technique has been disclosed in U.S. Pat. No. 4,855,888, “Constant frequency resonant power converter with zero-voltage switching” issued to Christopher P. Henze, Ned Mohan and John G. Hayes at Aug. 8, 1989, U.S. Pat. No. 5,442,540, “Soft-switching PWM converters” issued to Guichao C Hua and Fred C. Lee at Aug. 15, 1995, and U.S. Pat. No. 6,356,462 “Soft-switched full-bridge converters” disclosed by Yungtaek Jang and Milan M. Jovanovic at Mar. 12, 2002. In U.S. Pat. No. 5,973,939, “Double forward converter with soft-PWM switching” issued to F. Don Tan at Oct. 26, 1999 and U.S. Pat. No. 6,191,960, “Active clamp for isolated power converter and method of operating thereof” issued to Simon Fraidlin and Anatoliy Polikarpov at Feb. 20, 2001, the active clamp technique has been employed in the forward zero-voltage switching power converters. In U.S. Pat. No. 6,069,798, “Asymmetrical power converter and method of operation thereof” issued to Rui Liu at May 30, 2000, an asymmetrical scheme has been developed for a half-bridge (HB) topology.

[0005] Among various zero-voltage switching power converters, a parasitic leakage inductor of the transformer or at least one additional magnetic component is used as a resonant inductor or switch to generate a circulating current, so as to achieve the zero-voltage transition and switching. The parasitic leakage inductor of the transformer or the additional magnetic component, though aiding zero-voltage transition and switching, inevitably increases switching stress and noise. Further, in such an approach, power consumption caused by circulating current is significantly high in the light load or zero-load condition.

SUMMARY OF INVENTION

[0006] The present invention provides a zero-voltage switching pulse width modulation power converter for high frequency operation. The zero-voltage switching pulse width modulated power converter is operated at a constant frequency with low switching loss, low stress, and low noise.

[0007] The present invention further provides a zero-voltage switching pulse width modulation power converter that can generate zero-voltage transition and switching without using an additional magnetic device or leakage inductor of the transformer.

[0008] The present invention also provides a zero-voltage switching pulse width modulation power converter that consumes relatively low power in the light load and zero-load conditions.

[0009] Further, the present invention provides a control scheme to optimize soft switching of a power converter.

[0010] The zero-voltage switching pulse width modulation power converter provided by the present invention comprises a transformer, a secondary circuit, a pair of main switches and a pair of auxiliary switches. The transformer has a primary winding coupled to the main and auxiliary switches and a secondary winding coupled to the secondary circuit. The main switches and auxiliary switches intermittently conduct an input voltage source to the primary winding of the transformer, such that the soft-switching power converter is operated in four operation stages in each switching cycle.

[0011] In the first operation stage, the transformer is conducted to the input voltage source by switching on the main switches, such that power is delivered from the primary winding to the secondary winding. In the second operation stage, the connection between the input voltage source and the transformer established by the main switches is cut off, such that energy stored in the transformer is reset and freewheeled back to the input voltage source through the auxiliary switches. Meanwhile, energy stored in the secondary circuit is continuously output therefrom. In the third operation stage, the transformer operates as an inductor with the secondary winding thereof open circuited. The auxiliary switches are thus switched under a zero-current switching mode. In the fourth stage, energy stored in and magnetizing the transformer in the third operation stage flies back to the input voltage source through the main switches to achieve a zero-voltage transition.

[0012] In the above pulse width modulated soft-switching power converter, the main switches and the auxiliary switches are driven by a first switching signal and a second switching signal, respectively. The first and second switching signals are preferably in the form of pulse signals with a first pulse width and a second pulse width, respectively. Preferably, the first pulse width is broader than the second pulse width. In such manner, in each switching cycle, the main switches are switched on only in the first operation stage, and the auxiliary switches are switched on only in the third operation stage. In the second and fourth operation stages, both the main switches and auxiliary switches are switched off.

[0013] In the above pulse width modulated soft-switching power converter, the duration of the second operation stage can be extended, allowing the energy stored in the transformer to be completely released. The energy stored in the transformer in the third operation stage is equal to the multiplication of the square of the input voltage and the square of duration of the third operation stage divided by 2 times inductance of the primary winding. In order to achieve the zero-voltage transition in the fourth operation stage, the energy stored in the transformer in the third stage is no less than energy needed for charging parasitic capacitors of the main switches. Further, a minimum duration of the fourth operation stage is required to achieve zero-voltage transition. The minimum duration of the fourth operation stage, also referred as a minimum transfer time, is proportional to the inductance of the primary winding and inversely proportional to the parasitic capacitance of the main switches. Moreover, the fourth operation stage may be delayed by a delay time for switching on parasitic diodes of the main switches, allowing the energy stored in the transformer to flow back to the input voltage source, so as to achieve the zero-voltage transition. Therefore, the energy stored in the third operation stage is no less than the sum of the energy needed for charging the parasitic capacitors of the main switches and the energy needed during the delay time.

[0014] The pulse width modulated soft-switching power converter provided by the present invention further comprises a controller to generate the first switching signal and the second switching signal for driving the main and auxiliary switches, respectively. By the controller, a pulse width modulation switching frequency is determined. The pulse width modulated soft-switching power converter further comprises a first resistor coupled to the controller to adjust a pulse width modulation switching frequency. The pulse width modulated soft-switching power converter further comprises a second resistor coupled to the controller to adjust a pulse width of the second switching signal. The soft-switching power converter further comprises a third resistor coupled to the controller to adjust a pulse width of the second switching signal as a function of a load of the power converter.

BRIEF DESCRIPTION OF DRAWINGS

[0015] The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the present invention. In the drawings,

[0016]FIG. 1 is a circuit diagram of a soft-switching power converter according to the present invention;

[0017]FIG. 2 shows the waveforms in various operation stages of each switching cycle of the soft-switching power converter as shown in FIG. 1;

[0018]FIG. 3a shows the current flow of the soft-switching power converter as shown in FIG. 1 in a first operation stage of one switching cycle;

[0019]FIG. 3b shows the current flow of the soft-switching power converter as shown in FIG. 1 in a second operation stage of one switching cycle;

[0020]FIG. 3c shows the current flow of the soft-switching power converter as shown in FIG. 1 in a third operation stage of one switching cycle;

[0021]FIG. 3d shows the current flow of the soft-switching power converter as shown in FIG. 1 in a fourth operation stage of one switching cycle;

[0022]FIG. 4 shows a circuit that generates switching signals for controlling the main switches and the auxiliary switches of the soft-switching power converter as shown in FIG. 1; and

[0023]FIG. 5 shows the circuit for generating a programmable current supplied to the circuit as shown in FIG. 4.

DETAILED DESCRIPTION

[0024]FIG. 1 shows the topology of a soft-switching power converter provided by the present invention. As shown in FIG. 1, the soft-switching power converter comprises a transformer 50, a pair of main switches 10 and 20, a pair of auxiliary switches 30 and 40, and a secondary circuit. The transformer 50 further comprises a primary winding Wp coupled to the main and auxiliary switches 10, 20, 30 and 40 and a secondary winding Ws coupled to the secondary circuit. More specifically, in this embodiment, the main switch 10 connects the primary winding Wp to an input voltage source V_(IN) at a node A of a first end thereof, which is further connected to the auxiliary switch 40. The auxiliary switch 30 connects the input voltage source V_(IN) to a node B at a second end of the primary winding Wp, and the node B, is further connected to the main switch 20. The main and auxiliary switches 10, 20, 30 and 40 include metal-oxide semiconductor field effect transistors (MOSFET), insulated gate bipolar transistors (IGBT), or gate-turn-off transistors (GTO), for example. As shown in FIG. 1, the input voltage source V_(IN) is further connected to a capacitor 5.

[0025] The secondary circuit comprises a half-bridge rectifier, which is assembled of a diode 60 which often referred as the rectifying diode and a reverse or freewheel diode 70, an inductor 80, a capacitor 90, and an output terminal for outputting an output voltage V_(O). The positive terminal of the diode 60 is coupled to a first end of the secondary winding Ws, and a positive terminal of the diode 70 is coupled to a second end of the second winding Ws. The inductor 80 is connected between the negative terminals of the diodes 60 and 70 and the output terminal of the secondary circuit. The output capacitor 90 has one terminal connected to the positive terminal of the freewheel diode 70 and the other end connected between the inductor 80 and the output terminal of the secondary circuit.

[0026] As shown in FIG. 1, the main switches 10 and 20 are driven by a switching signal S₁ while the auxiliary switches 30 and 40 are driven by a switching signal S₂. Referring to FIG. 2, the switching signal S₁ preferably has a pulse waveform with a pulse width of T₁, while the switching signal S₂ is preferably in a pulse waveform with a pulse width of T₃.

[0027] By controlling the on/off status of the main and auxiliary switches 10, 20, 30 and 40, the power converter as shown in FIG. 1 has four operation stages for each switching cycle as shown in FIGS. 3a to 3 d. In order to operate the power converter in four operation stages, the switching signals S₁ and S₂ are out of phase. That is, in the embodiment as shown in FIG. 1 and FIG. 2, the main switches 10 and 20 are turned on when the switching signal S₁ is high within a duration T₁ for each switching cycle. During the duration T₁, the auxiliary switches 30 and 40 are in an off state. When the switching signal S₁ has been low for a period of time, that is, as shown in FIG. 2, for T₂, the auxiliary switches 30 and 40 are turned on within duration T_(3.)

[0028] The four operation stages are further described as follows with reference to FIGS. 2 and 3a to 3 d. At the beginning of each switching cycle, as shown in FIG. 2, the main switches 10 and 20 are switched on within the pulse width T₁ of the switching signal S₁. As the main switches 10 and 20 are activated, as shown in FIG. 3a, the current I₁ flows from the input voltage source V_(IN) through the main switches 10 and 20 across the primary winding Wp. Therefore, the input voltage source V_(IN) is applied to the primary winding Wp. The polarities of the primary winding Wp and the secondary winding Ws conduct the diode 60 by supplying a forward bias thereto. Meanwhile, the diode 70 is cut off by being reversely biased. Therefore, a secondary current I₂ is induced to flow through the diode 60 and the inductor 80 along the arrow as shown in the secondary circuit of FIG. 3a. Consequently, the energy is delivered to the output terminal to result in an output voltage V_(O).

[0029] After T₁, the switching signal S₁ drops to zero or a lower voltage to switch off the main switches 10 and 20 in the second operation stage as shown in FIG. 2. Referring to FIG. 2 and FIG. 3b, the primary current I₁ is cut off. However, before the auxiliary switches 30 and 40 are switched on by the pulse of the switching signal S₂, the energy that stored in the primary winding Wp produces a current that inverts the polarity of the primary winding Wp and the secondary winding Ws and induces a current I₃ flowing back to the input voltage source V_(IN). As a result, the diode 60 in the secondary winding Ws is reversely biased and cut off, and the secondary winding Ws become an open circuit. Therefore, the energy stored in transformer 50 (primarily generated by leakage inductance of the transformer 50) is reset and freewheeled back to the input voltage source V_(IN) with the current I₃ flowing through the parasitic diodes of the auxiliary switches 30 and 40. Meanwhile, the diode 70 is forwardly biased and conducted to form a closed loop between the diode 70, the inductor 80, and the capacitor 90 with a current I₄ circulating therethrough. Therefore, the energy stored in the inductor 60 and the capacitor 70 is thus continuously delivered to the output terminal of the secondary circuit. Further, as shown in FIG. 2, the duration T₂ of the second operation stage is varied according to the amount of energy stored in the transformer. The variable duration of the second operation stage is indicated as T_(R) in FIG. 2.

[0030]FIGS. 2 and 3c show the third operation stage in each switching cycle of the soft-switching power converter. As shown in FIG. 2, before the next switching cycle, that is, before the main switches 10 and 20 are switched on by the pulse of the switching signal S₁ again, the pulse of the switching signal S₂ switches on the auxiliary switches 30 and 40 within the duration T₃ thereof. As shown in FIG. 3c, by switching on the auxiliary switches 30 and 40, the input voltage source V_(IN) is connected to the node B to induce a current I₅ across the primary winding Wp and directed along at the side of the primary winding Wp, and energy is stored in the transformer 50. Similar to the second operation stage, polarity of the transformer 50 results in reverse bias of the diode 60, so that the secondary winding Ws becomes an open circuit. The transformer 50 thus operates as an inductor in the third operation stage, such that the power converter is operated similar to a discontinuous mode flyback power converter. Switching on the auxiliary switches 30 and 40 under zero-current switching (ZCS) or zero-voltage switching (ZVS) can thus be realized. The energy stored in the transformer 50 in the third operation stage can be expressed as:

ε=Lp×Ip ²/2,

[0031] where Lp is the inductance of the primary winding Wp, Ip is the current flowing through the primary winding Wp and can be expressed as:

Ip=V _(IN) ×T ₃ /Lp,

[0032] where T₃ is the turn-on time of the auxiliary switches 30 and 40, that is, the pulse width of the switching signal S₂. By substituting the equation of Ip into the equation of the energy ε,

ε=V _(IN) ² ×T ₃ ²/(2×Lp).

[0033] Therefore, the energy stored in the transformer 50 in the third operation stage is proportional to the multiplication of the square of the input voltage V_(IN) and the square of the pulse width T₃ of the switching signal S₂, and inversely proportional to the inductance of the primary winding Wp.

[0034] In the fourth operation stage as shown in FIG. 2 and FIG. 3d, the switching signal S₂ drops to zero or lower to switch off the auxiliary switches 30 and 40, while the switching signal S₁ stays zero or lower to keep the main switches 10 and 20 off. The current I₅ produced in the third operation stage flows through the primary winding Wp. Meanwhile, the energy stored in the transformer 50 during the period T₃ of the third operation stage flies back to the input voltage source V_(IN) through the parasitic diodes of the main switches 10 and 20 to achieve a zero-voltage transition.

[0035] To turn on the parasitic diodes of the main switches 10 and 20, the parasitic capacitors of the main switches 10 have to be discharged in advance. In other words, the zero-voltage transition is achieved when the parasitic capacitors of the main switches 10 and 20 have been discharged. Therefore, to achieve the zero-voltage transition, the energy stored in the transformer 50 in the third operation stage has to be larger than the energy needed to discharge both of the parasitic capacitors of the main switches 10 and 20. The relation can be expressed by the following inequality:

V _(IN) ² ×T ₃ ²/(2×Lp)>2×(Cr×V _(IN) ²/2)

[0036] Where Cr is the parasitic capacitance of the main switch 10 or 20. As the resonant frequency fr between the primary winding Wp and the parasitic capacitors of the main switches 10 and 20 at the period T₃ can be expressed as:

fr=1/(2π×(Lp×Cr)^(1/2))

[0037] the minimum transfer time T_(F) to achieve phase shift for zero-voltage transition is

T _(F)=1/(4×fr)=π×(Lp×Cr)^(1/2)/2.

[0038] That is, the minimum time from the switching signal S₂ dropping to low to switch off the auxiliary switches 30 and 40 to the time the main switches 10 and 20 being switched on again by the pulse T₁ of switching signal S₁, namely, the minimum duration of the fourth stage can be calculated by the above equation of T_(F). From the above equation, it is known that the minimum time required for achieving the zero-voltage transition is determined by the inductance of the primary winding Wp and the parasitic capacitance Cr.

[0039] The duration of the fourth stage may be delayed by a delay time T_(Z) after the parasitic diodes of the main switches 10 and 20 are conducted and before starting the next switching cycle. Therefore, the total duration of the fourth stage is the sum of the minimum transfer time T_(F) and the delay time T_(Z), that is, T₄=T_(F)+T_(Z). However, in order to operate the inductor 80 in a continuous mode under the condition of zero-voltage transition, the energy stored in the transformer 50 in the duration T₃ of the third operation stage must satisfy the following inequality:

V _(IN) ² ×T ₃ ²/(2×Lp)>{[Cr×VIN ² ]+[V _(IN)×(Ns/Np)×I _(O) ×T _(Z) ]+[T _(Z) ×V _(IN) ² ×T ₃ /Lp]},

[0040] where Ns and Np are turns of the secondary and primary windings Ts and Tp, respectively, and I_(O) is the output current of the power converter. That is, the energy stored in the transformer 50 in the duration T₃ has to be large enough to discharge the parasitic capacitance 2Cr, and then provide the primary side backward freewheeling current and sustain the output current during the delay time T_(Z).

[0041]FIG. 4 shows a circuit diagram of the controller, which generates the switching signals S₁ and S₂. As shown in FIG. 4, the controller includes an oscillator 200, an inverter 370, comparators 320 and 330, a programmable current source 310, D-type flip-flops 340, 350 and 360, and AND gates 380 and 390. The oscillator 200 is coupled to the input of the inverter 370, the negative inputs of the comparators 320 and 330, and a reference resistor 515. The output of the inverter 370 is coupled to the D-type flip-flops 340, 350, 360, and the inputs of the AND gates 380 and 390. The D-type flip-flop 340 is further coupled to a voltage source Vcc and the output of the comparator 320, while the output thereof is coupled to the AND gate 380. Signals S_(A) and S_(B) output by the D-type flip-flop 350 are inverted from each other and fed into the AND gates 380 and 390, respectively. Signal S_(B) is fed to the D-type flip-flop 350. The D-type flip-flop 360 is further coupled to the output of the comparator 330 and the voltage source Vcc, while the output thereof is coupled to the input of the AND gate 390. From the AND gates 380 and 390, the switching signals S₁ and S₂ are output to drive the main switches 10, 20 and the auxiliary switches 30, 40, respectively.

[0042] As shown in FIG. 4, the D-type flip-flop 350 provides inverted signals S_(A) and S_(B) to the AND gates 380 and 390, respectively. The main switches 10, 20 and the auxiliary switches 30 and 40 as shown in FIG. 1 are driven out of phase with slightly less than 50% of the maximum duty cycle. The oscillator 200 is operative to generate a clock signal 210, a ramp signal 220 and a saw signal 230. The clock signal 210 is input to the inverter 370 to determine the switching frequency. A feedback voltage V_(FB) reflecting the output voltage of the power converter is compared to the ramp signal 220 generated by the oscillator 200 in the comparator 320. When the feedback voltage V_(FB) is high, the pulse width T₁ of the switching signal S₁ is broadened, and more power is forwarded to the output of the power converter. Therefore, the feedback voltage V_(FB) sourced from the output voltage V_(O) of the power converter is used to regulate the output voltage V_(O). The oscillator 200 further generates a saw signal 230 that is synchronized with the ramp signal 220. The amplitude of the saw signal 230 is inversely proportional to that of the ramp signal 220. The programmable current source 310 generates a programmable current Im as a function of the feedback voltage V_(FB). The programmable current Im flows through a resistor 315, and thus results in a programmable voltage across the resistor 315. The saw signal 230 is compared to the current Im in the comparator 330. By adjusting the current Im, the voltage across the resistor 315 is programmed, such that the pulse width T₃ of the switching signal S₂ can be programmed or adjusted. When the current Im is increased, the pulse width T₃ of the switching signal S₂ is broadened and zero voltage switching can be achieved.

[0043]FIG. 5 shows the circuit of the programmable current source 310. As shown in FIG. 5, the programmable current source 310 includes a current source 490, a pair of mirrored transistors 460 and 470, another transistor 450, op-amplifiers 410 and 420 and a resistor 415. The current source 490 is connected to the constant voltage source V_(CC) to provide a constant source current. In the programmable current source 310, the current Im can be derived as:

Im=K×(V _(FB) −V _(TH))/Rm,

[0044] where 0<Im<Imax.

[0045] In the above equation and inequality of the adjustable current Im, Rm is the resistance of the resistor 415, K is the mirror ratio of the mirrored transistor 460 and 470, Imax=Ic−Ib, where Ic is the current of the constant current source 490, and Ib is the current flowing through the transistor 450. Thus constructed, the resistor 415 determines the variation range programmed by the feedback voltage V_(FB). The pulse width T₃ of the switching signal S₂ becomes narrower or even turned off when the feedback V_(FB) is reduced, that is, when the load coupled to the output of the power converter is decreased. Therefore, no circulated power is consumed, and power consumption in the light load condition is reduced.

[0046] In the topology of the soft-switching power converter provided by the present invention, the main switches and the auxiliary switches are activated with zero voltage switching and zero current switching operations, respectively. Compared to the conventional pulse width modulation converter, the switching loss is greatly reduced. Further, the present invention does not require an additional magnetic device or leakage inductance of the transformer, such that the switching loss, stress and noise are reduced. In addition, the power consumption under light load condition is reduced.

[0047] While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims. 

1. A pulse width modulated soft-switching power converter, comprising: a transformer, having a primary winding and a secondary winding; a secondary circuit, coupled to the secondary winding; and a pair of main switches and a pair of auxiliary switches coupled to the primary winding, the main switches and auxiliary switches being operative to intermittently conduct an input voltage source to the primary winding of the transformer to operate the soft-switching power converter in four operation stages in each switching cycle; wherein power is delivered from the primary winding to the secondary winding in a first operation stage for each switching cycle; energy stored in the transformer is reset and freewheeled back to the input voltage source through the auxiliary switches, and energy stored in the secondary circuit is continuously output in a second operation stage for each switching cycle; the transformer operates as an inductor with the secondary winding open circuited, so as to realize a zero-current switching mode of the auxiliary switches in a third operation stage for each switching cycle; and energy stored in and magnetizing the transformer in the third operation stage flies back to the input voltage source through the main switches to achieve a zero-voltage transition in a four operation stage for each switching cycle.
 2. The pulse width modulated soft-switching power converter as recited in claim 1, wherein: the main switches are switched on and the auxiliary switches are switched off in the first operation stage; the main switches are switched off and the auxiliary switches remains off in the second operation stage; the main switches remain off and the auxiliary switches are switched on in the third operation stage; and the main switches remains off and the auxiliary switches are switched off in the fourth operation stage.
 3. The pulse width modulated soft-switching power converter as recited in claim 1, wherein duration of the second operation stage is varied until the energy stored in the transformer is completely released.
 4. The pulse width modulated soft-switching power converter as recited in claim 1, wherein energy stored in the transformer in the third stage is equal to multiplication of a square of the input voltage and a square of duration of the third operation stage divided by 2 times inductance of the primary winding.
 5. The pulse width modulated soft-switching power converter as recited in claim 1, wherein energy stored in the transformer in the third operation stage is no less than energy generated by parasitic capacitors of the main switches to achieve the zero-voltage transition in the fourth operation stage.
 6. The pulse width modulated soft-switching power converter as recited in claim 1, wherein a minimum duration of the fourth operation stage to achieve zero-voltage transition is proportional to the inductance of the primary winding and inversely proportional to the capacitance of the parasitic capacitors of the main switches.
 7. The pulse width modulated soft-switching power converter as recited in claim 1, wherein the duration of the fourth operation stage is delayed by a delay time for turning on parasitic diodes of the main switches.
 8. The pulse width modulated soft-switching power converter as recited in claim 7, wherein the energy stored in the third operation stage is no less than the sum of the energy stored in the parasitic capacitors of the main switches and the energy generated during the delay time.
 9. The pulse width modulated soft-switching power converter as recited in claim 1, wherein the main switches and the auxiliary switches are selected from a group consisting of metal-oxide semiconductor field effect transistors (MOSFET), insulated gate bipolar transistors (IGBT) and gate-turn-off transistors (GTO).
 10. The pulse width modulated soft-switching power converter as recited in claim 1, wherein the main switches are driven by a first switching signal with a first pulse width, and the auxiliary switches are driven by a second switching signal with a second pulse width.
 11. The pulse width modulated soft-switching power converter as recited in claim 10, wherein the first pulse width is broader than the second pulse width.
 12. The pulse width modulated soft-switching power converter as recited in claim 1, wherein the secondary circuit further comprises: a first diode with a positive terminal coupled to a first end of the secondary winding; a second diode with a positive terminal coupled to a second end of the secondary winding; an inductor, connected between negative terminals of the first and the second diodes and an output terminal of the secondary circuit; and a capacitor, connected between the inductor and the positive terminal of the second diode.
 13. The pulse width modulated soft-switching power converter as recited in claim 12, wherein the first diode is conducted, and the second diode is cut off in the first operation stage.
 14. The pulse width modulated soft-switching power converter as recited in claim 12, wherein the first diode is cut off, and the second diode is conducted in the second operation stage.
 15. The pulse width modulated soft-switching power converter as recited in claim 12, wherein the first diode is cut off to open the secondary winding in the third operation stage.
 16. The pulse width modulated soft-switching power converter as recited in claim 1, further comprising a controller operative to generate a first switching signal and a second switching signal to drive the main and auxiliary switches, respectively.
 17. The pulse width modulated soft-switching power converter as recited in claim 16, wherein the first and second switch signals include pulse signals.
 18. The pulse width modulated soft-switching power converter as recited in claim 16, further comprising a resistor coupled to the controller to adjust a pulse width modulation switching frequency.
 19. The pulse width modulated soft-switching power converter as claimed in claim 16, further comprising a resistor coupled to the controller to adjust a pulse width of the second switching signal.
 20. The soft-switching power converter as claimed in claim 16, further comprising a resistor coupled to the controller to adjust a pulse width of the second switching signal as a function of a load of the power converter.
 21. The soft-switching power converter as claimed in claim 16, wherein the controller further comprises: an oscillator, operative to generate a clock signal, a ramp signal and a saw signal; an inverter, with an input terminal receiving the clock signal and an output terminal; a first comparator, with a positive terminal connected to a feedback voltage obtained from the secondary circuit, a negative terminal coupled to the ramp signal, and an output terminal; a second comparator, with a positive terminal coupled to a variable current, a negative terminal coupled to the saw signal, and an output terminal; a first D-type flip-flop, coupled to the output terminals of the inverter and the first comparator and a voltage source, the first D-type flip flop further comprising an output; a second D-type flip-flop, coupled to the output terminals of the inverter and the second comparator and the voltage source, the second D-type flip-flip further comprising an output; a third D-type flip-flop, coupled to the output terminal of the inverter, the third D-type flip-flop having a first output and a second output inverted from the first output; a first AND gate, coupled to the outputs of the first D-type flip-flop and the inverter, and a first output of the third D-type flip-flop; and a second AND gate, coupled to the outputs of the second D-type flip-flop and the inverter, and the second output of the third D-type flip-flop.
 22. The pulse width modulated soft-switching power converter as recited in claim 21, wherein the first AND gate is operative to generate a first switching signal to drive the main switches, and the second AND gate is operative to generate the second switching signal to drive the auxiliary switches.
 23. The pulse width modulated soft-switching power converter as recited in claim 21, further comprising a programmable current source to generate the variable current, wherein the variable current is adjusted in response to the feedback voltage.
 24. The pulse width modulated soft-switching power converter as recited in claim 23, wherein the programmable current source further comprises: a constant current source; a pair of mirrored transistors connected to the constant current source; a transistor coupled to one of the mirrored transistors; a first op-amplifier coupled between the transistor and the feedback voltage; and a resistor, coupled to the transistor and the first op-amplifier; and a second op-amplifier coupled to the resistor and a threshold voltage.
 25. The pulse width modulated soft-switching power converter as recited in claim 24, wherein the programmable current source generating the variable current is proportional to a mirror ratio of the pair of mirrored transistors and the difference between the feedback voltage and the threshold voltage, and inversely proportional to resistance of the resistor.
 26. A power converter, comprising: a transformer, having a primary winding and a secondary winding; a first main switch, connected between an input voltage source and a first end of the primary winding and driven by a first pulse signal; a second main switch, connected to a second end of the primary winding and driven by the first pulse signal; a first auxiliary switch, connected between the input voltage source and the second end of the primary winding and driven by a second pulse signal; a second auxiliary switch, connected to the first end of the primary winding and driven by the second pulse signal; and a secondary circuit, connected between the secondary winding and an output of the power converter; wherein the first pulse signal has a pulse width wider than that of the second pulse signal; and the first pulse signal and the second pulse signal are out of phase.
 27. The power converter as recited in claim 26, wherein the secondary circuit further comprises: a half-bridge rectifier coupled to the secondary winding; an inductor coupled to the half-bridge rectifier; and a capacitor coupled to the inductor.
 28. A switching device, suitable for switching a soft-switching power converter operated with a constant high frequency, wherein the power converter comprises a transformer with a primary winding and a secondary winding, the switching device comprises: a first switch assembly, operative to periodically conduct the primary winding to an input voltage source from a first end of the primary winding; a second switch assembly, operative to periodically conduct the primary winding to the input source from a second end of the primary winding; and a half-bridge rectifying circuit, operative to conduct the secondary winding to an output when the first end of the primary winding is conducted to the input voltage source, and opens the secondary winding when the first switch assembly cuts off the connection.
 29. The switching device as recited in claim 28, wherein the half-bridge rectifying circuit further comprises an inductor and a capacitor.
 30. The switching device as recited in claim 28, further comprising a controller operative to generate a first switching signal driving the first switch assembly and a second switching signal driving the second switch assembly.
 31. The switching device as recited in claim 30, wherein the first switching signal and the second switching signal are out of phase from each other.
 32. The switching device as recited in claim 30, wherein the first switching signal has a first pulse width wider than that of the second switching signal.
 33. The switching device as recited in claim 29, further comprising a feedback loop operative to supply a feedback voltage, which reflects an output voltage of the power converter to the controller. 